1. Field of the Invention
This invention relates to the making of steel, and in particular, it relates to the making of oxide inclusion free low-carbon steels with desirably low nitrogen content. Still more particularly, it relates to the making of steels of the kind indicated above which contain about 1.0 to 4.5 percent of silicon as an alloying element and are intended for use in sheet or strip form in the electrical industry in applications in which the magnetic properties of the steel are important.
In one aspect of the invention, the invention relates to a composition of matter in the nature of a "synthetic slag material" or a "synthetic flux material", and to the method in which that composition of matter is used especially during bubbling of an inert gas in the making of steel of the kind indicated above.
In another aspect, the invention is particularly concerned with a method wherein the use of the composition of matter mentioned above is combine with certain other measures and expedients in the processing of the steel during the stages thereof between the completion of the blowing with oxygen in an oxygen steelmaking vessel and the pouring of the steel into an ingot mold or a continuous-casting machine. In other words, the invention is concerned with the in-ladle treatment of a ferrous melt in order to bring it to a desirable chemistry, temperature, and degree of homogeneity to suit it for the subsequent pouring or casting step.
2. Description of the Prior Art
There may be taken as known a practice for making a silicon electrical steel of the kind practiced by the applicants' assignee for several months before the present invention was made. The prior practice is useful for producing grain-oriented silicon steel generally, which steel melt may initially contain the nominal compositions of:
______________________________________ C Mn S Si Cu Fe ______________________________________ 0.03 0.07 0.03 3.2 0.2 Balance ______________________________________
and impurities, where the impurities may include on the order of 10 parts per million aluminum, 15 parts per million titanium, 50 parts per million oxygen, and 55 parts per million nitrogen.
In accordance with the previous practice mentioned above, it is a known practice to desulfurize the steel by injecting calcium carbide in a hot metal ladle, and then blowing a mixture of desulfurized hot metal and scrap in a basic oxygen furnace to an aim turn-down composition of 0.025 to 0.035 percent carbon and an aim turn-down temperature. About 80 tons of this low-carbon steel from the BOF is then tapped into a zoned ladle containing an appropriate amount, approximately 7500 pounds, of a low-aluminum, low-titanium ferrosilicon, then bubbled with argon lance or bubbler at a suitable flow rate for a sufficient time to achieve improvements in cleanliness of the melt. Additions of manganese, carbon, sulphur, and copper are made in the ladle, as necessary, based upon the observed chemistry of a sample taken after tapping from the basic oxygen furnace. In this prior practice, there is no purposeful addition of any flux material to the ladle. During the bubbling, there is generated in the ladle a slag which has relatively high contents of alumina (8 percent) and titania (0.5 percent).
By zoned ladle, it is meant that the ladle or vessel is lined with various grades of alumina-silica refractory on the bottom and sidewalls. For example, a fireclay having 40 to 60 percent alumina-balance silica may be used to line a vessel, except for a zone adjacent a slag layer which zone may have a refractory having 80 percent alumina-balance silica.
Those skilled in the art have long been aware of the desirability of getting and keeping the total oxygen content, dissolved oxygen plus oxides of steel as low as possible.
A low-carbon steel (&lt;0.05%) coming out of an oxygen-blown furnace (such as a BOF), contains large amounts of dissolved oxygen (&gt;0.05%). The dissolved oxygen in the steel is then removed (deoxidized) by adding one or more deoxidizers such as Al, Si, Mn, etc., depending on the grade of the steel to be made. The deoxidizers remove the dissolved oxygen by forming oxides which ideally should float out of the melt and be absorbed into the slag phase. However, the flotation and absorption of these oxides is always incomplete and they remain suspended in the melt, resulting in a total oxygen which is much greater than the dissolved oxygen.
Oxygen, if not removed, tends to react with other elements present, like the iron, silicon, aluminum, and titanium, to form inclusions and create a "dirty steel". It is especially difficult to obtain desirably low dissolved-oxygen contents in the low-carbon steels, the steels containing about 0.05 percent of carbon or less. With steels richer in carbon, the carbon-oxygen reaction drives dissolved oxygen out of the melt, but in low-carbon steels, this reaction does not occur to any so great an extent. Thus, when product steels are desired that need to be low in carbon (like the silicon electrical steels, where high carbon contents are associated with greater core losses and/or other impaired electrical or magnetic properties), obtaining a desirably lower content of dissolved oxygen poses a more difficult problem.
With the practice indicated above, using the zoned ladle, it was found that whenever measures were taken to lower the oxide inclusions in the melt, the dissolved oxygen also got slightly lowered and then there would be obtained a steel undesirably higher in its contents of aluminum and titanium. What happens in this process is controlled or greatly influenced by the equilibrium between the chemical composition of the slag and the chemical composition of the underlying steel.
In the case of the above-mentioned previous practice with the use of a zoned ladle, there was generated a slag containing about 28 percent lime, 48 percent silica, 8 percent alumina, 0.5 percent titania, 10 percent magnesia, and 1 percent manganese oxide.
A ladle entirely lined with 80 percent alumina instead of the zoned ladle has been used, and there was then obtained a slag composition which was essentially the same, except for a slightly lower (0.3 percent) content of titania. With the higher alumina-lined ladle, there were the same problems of too-high aluminum and titanium contents, if the dissolved-oxygen content got slightly lowered.
There are, in the prior art, various known methods and apparatus for treating molten steel to remove dissolved gases or to remove nonmetallic inclusions there, such as the Dortmund-Horder, RH and VOD processes, or other methods such as the use of electric arc ladle furnaces, ASEA-SKF ladle furnaces and the like; the principal drawback of these processes and methods is that they are relatively costly to practice.
There can be taken as belonging to the prior art a commercially available typical calcium silicate composition containing about 50 percent silica, 47 percent lime, and small amounts of various impurities.
Such material has a melting point of about 2811 degrees Fahrenheit, and a bulk density of approximately 80 pounds per cubic foot, and it is available at a cost sufficiently low that it can be used in substantial quantities without causing the steelmaking process to become uneconomical. The prior art has not contained, however, any particular teachings or suggestions about how to use such a material to obtain the favorable results that are available with the present invention; in fact, our own first several experiences with trying to use such a material, which were in ways not in accordance with the present invention, did not yield the desired results.
It may be taken that the addition of fluorspar (calcium fluoride), as an agent for making a slag less viscous, is well known. Substitutes or equivalents for fluorspar are known to those skilled in the art.
There is a body of prior art which concerns the chemical compositions of the refractory materials used to line vessels for holding molten ferrous metals, and the chemical compositions of the slags which form (or are provided) on top of the molten ferrous metals. The refractory materials may be acidic, like silica brick, or basic, like dolomite, or more nearly neutral, like alumina or fireclay. A slag composition may likewise be characterized as acidic or basic, largely in accordance with the relative proportions of the acid-forming and base-forming oxides present. Slags richer in silica and in iron oxide are more acidic; slags richer in lime or magnesia are more basic. It is known that it is important to avoid having a slag too acidic in a vessel lined with a basic refractory, or vice versa, because this leads to having the slag attack the lining the shorten its service life. Iron-refining processes conducted with a slag which is basic, rather than acidic, do a better job of removing sulphur and phosphorus from the molten ferrous material.
Those skilled in the art are aware that in the step following the blowing with oxygen, the step of making a ladle addition of enough ferrosilicon to get the composition of the steel up of the level of silicon content desired for an electrical steel, about 1.0 to 4.5 percent by weight, hardly any slag is formed naturally, except to the extent that the molten metal comes into contact with air or oxygen, either dissolved oxygen or a combined oxygen in the form of some metal oxides. The molten metal will, in the absence of a slag covering, not only react readily with any available oxygen to form metal oxides but also pick up nitrogen readily if it comes into contact with air. Bubbling with argon is practiced in order to provide adequate mixing of the ferrosilicon with the ferrous melt removed from the oxygen vessel and, primarily to float out silicon oxide inclusions from the ferrous melt, formed as a result of silicon deoxidation.
The step of lancing or bubbling with an inert gas, particularly argon, in a ladle covered with a suitable lid or cover, during the making of a homogeneous melt from the ferrosilicon and the blown metal removed from the basic oxygen furnace, is old well known. By inert gas it is meant any gases which are chemically inactive or have permissibly low activity in the melt and may be used.
There are prior art references which relate to calcium aluminate synthetic slags or relate to the use of synthetic-flux or synthetic-slag material which are introduced by lancing, that is, conveying the material into the steel in the ladle by means of a stream of carrier gas. These include the following items:
1. K. Narita et al., Trans. ISIJ, page B-112, Vol. 20, No. 4, 1980. PA0 2. E. T. Turkdogan, Ironmaking and Steelmaking, page 64, Vol. 12, No. 2, 1985. PA0 3. H. Saito et al., Trans. ISIJ, page B-345, Vol. 22, 1982. PA0 4. A. Ishi et al., Ladle Metallurgy Principles and Practices, Published by Iron and Steel Society of AIME, Edited by R. J. Fruehan, page 137, 1985. PA0 5. T. Takenouchi et al., Trans. ISIJ, page 758, Vol. 19, 1979. PA0 6. Start up and Operation of USS Lorain-Cuyahohga Works CAB Ladle Treatment Facility, Public Document, Reprint Available. PA0 7. D. J. Diederich et al. "Improving Internal Cleanliness For Bar and Rod Products", United States Steel Publication, Lorain, Ohio. PA0 8. A. Moriya et al., SCANINJECT III, Part III, page 32:1, 1983. PA0 9. J. G. Yount and R. J. Zaranek, "Steelmaking Proceedings", page 194, Vol. 64, 1981.